The present invention relates to an output control device for an electrical apparatus, and more particularly to an output control device for stabilizing a laser output.
For an electrical apparatus adapted to output optical energy, electrical energy, or mechanical energy, it is necessary that the output be stably controlled. Especially, it is essential for a high power laser such as a solid-state laser or a gas laser to have an output control device which stabilizes the output and holds its magnitude to a predetermined value. For instance, in a gas laser of the type in which the electric discharge caused in the laser tube is utilized to induce a laser beam, the discharge current is controlled in order to maintain the laser output at a desired value. In a laser such as a laser of the type in which a laser beam is induced by light emitted by a discharge tube such as a krypton lamp, the discharge current of the discharge tube is utilized to maintain the laser output at a desired value. These output control devices generally employ a negative feedback circuit.
An example of an output control device for a laser is shown in FIG. 2. In FIG. 2, reference numeral 10 designates a signal setting circuit for setting the level of a signal P at an optional value; 12, a subtraction circuit; 14, a multiplication circuit having a multiplication factor G; 16, an addition circuit; 18, a laser device; and 20, a laser power meter. The set value P is a target value for the output of the laser device 18.
If the output of the laser power meter 20 is represented by M, then the output of the subtraction circuit 12 is (P-M), and the output X of the addition circuit 16 is: EQU X=P+G(P-M) (1)
In general, EQU M=AX (2)
where A is a conversion factor dependent on the efficiency of the laser device 18 and other coefficients. As is apparent from equations (1) and (2), the output M of the laser device is stably controlled with respect to the set value P.
Recently, digital circuits have been extensively utilized to form various arithmetic circuits. In the above-described conventional device, digital operations are carried out in the setting circuit 10, the subtraction circuit 12, the amplifier 14, and the addition circuit 16. In the laser device, electric power corresponding to a digital value X is applied to the laser medium by an internal electric power source. The values P, X and M are all provided as digital values.
On the other hand, digital arithmetic circuits unavoidably include quantizing errors. Therefore, in order to ensure a desired accuracy for digital operations employing negative feedback in the presence of a digital error .DELTA.P, the arithmetic accuracy of the negative feedback circuit, i.e., the number of bits of the various parameters, must be determined for the minimum set value P. However, if such a method is employed, excessively high accuracy operations must be carried out for intermediate to the maximum set values of P, with the results that the arithmetic calculation time is long and the arithmetic circuits are considerably expensive.